Films of silica (SiO2) are used as electrically insulating coatings in various fields, because they comprise an inorganic oxide, may be excellent in the heat resistance, electrical insulation and the like, and flat films thereof may easily be obtained. Further, the silica film may generally be produced by using a vapor-phase process such as PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition), or a liquid-phase process such as sol-gel method.
In connection with the recent requirements for a sophisticated arrangement of electronic devices and/or electronic components, the intensity of an electric field to be used therein may also tend to be increased. A higher insulating property may be required in such an arrangement of devices or electronic components involving a high electric field intensity and therefore, the thickness of the silica film should be increased to a higher level.
However, in any of the above-described production processes such as vapor-phase process and liquid-phase process, it may be difficult to form a thick silica film, and usually, when the film thickness is 1 μm or more, crack formation is more liable to occur. The reason therefor may be considered because the Young's modulus of the “silica” itself constituting the film is high, and it is difficult for the film to conform to and relieve the stress, with respect to the internal stress generated during the film formation process due to a difference in the thermal expansion rate from the substrate material, or the internal stress generated due to the shrinkage of the film itself.
Further, in the case of an electronic device capable of deforming into a curve or the like, represented by an electronic paper, a silica insulating coating having a low flexibility (that is, having a high Young's modulus) can hardly conform to the deformation thereof as mentioned above, and therefore, it has generally been considered that a silica insulating coating may be improper for such an electronic device capable of being deformed.
As a measure for solving the above-described problem, Non-Patent Documents 1 and 2 propose an organic-modified silicate film, where an organic group is introduced into the siloxane network of the silica. They also disclose that, in a film made of a material also called, for example, an organic-inorganic hybrid (inorganic-organic hybrid), Ormosils or Ceramers, when an organic group such as methyl group is introduced into the siloxane network, the rigidity of the siloxane network may be relieved and the Young's modulus thereof may be decreased, whereby a film deposition even to a film thickness of 1 μm or more can be achieved without causing a crack therein.
Patent Document 1 discloses that such an organic-modified silicate film may be produced by using an organoalkoxysilane RxSi(OR′)4-x (wherein R is an organic group, OR′ is an alkoxy group, and x is 1, 2 or 3) as a starting material, by a method of forming a structure where an organic group R is introduced into the siloxane network through a hydrolysis/polycondensation reaction (generally, referred to as “sol-gel method”); that among structures having an organic group R which has been introduced into the siloxane network, a structure having a diorganosiloxane —O—Si(R)2—O— where two organic groups are bonded to Si may further have a flexibility and may be more preferred for the above-described purpose; that in particular, dimethylsiloxane —O—Si(CH3)2—O— where the organic group R of the diorganosiloxane above is a methyl group may be more preferred in the flexibility and also excellent in the heat resistance; that the structure containing dimethylsiloxane may include a structure which has been obtained by using a polydimethylsiloxane X-[—Si(CH3)2—O]m—Si(CH3)2—X (wherein X is a reactive functional group, and m is the number of units of the polydimethylsiloxane) as the starting material, by reacting it together with a metal alkoxide M(OR)n (wherein n is the number of alkoxy groups and is usually the valence of M); and that above all, when the mass-average molecular weight Mw of the polydimethylsiloxane is 900 or more, the production of a thick film of 1 μm or more may be facilitated, to thereby easily provide an insulating coating having a flexibility which is high enough to conform to the deformation of the substrate.
Patent Document 2 discloses, as a material which is analogous to the organic-modified silicate as described above, a silicone resin composition formed from a chain silicone oligomer such as polydimethylsiloxane, a metal alkoxide and an inorganic filler to be used for an insulating coating layer of a heat-resistant insulated electric wire.
Patent Document 3 discloses, in relation to a thin-film solar cell substrate, a method for forming an insulating coating formed on the surface thereof and having an irregular structure so as to enhance the light trapping efficiency, where the surface irregular structure is formed by utilizing a phase separation into a hydrophobic phase and a hydrophilic phase so as to provide a film to be formed from a polydimethylsiloxane and a metal alkoxide.
Patent Document 4 discloses an insulating coating having a flattened the surface, which has been formed from a poly(di-organo)siloxane and a metal alkoxide. It also discloses that the insulating coating is a film which is excellent in the flatness and therefore, may suitably be used as a coated substrate having an insulating coating with a large thickness to be used for an electronic device such as thin-film transistor, liquid crystal display, organic EL display and electronic paper.